Broad band hydrophone

ABSTRACT

A low frequency broad band hydrophone with orthogonal dipole outputs comprising a mass controlled housing dimensionally much smaller than a wavelength over the desired frequency band and having four piezoelectric elements cantilever-mounted radially to a center mass suspended within the housing is described. The velocity amplitude of the housing vibration is both independent of frequency and in phase with the acoustic pressures; the detection of said velocity amplitude by said elements provides orthogonal dipole outputs. The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. This invention relates to hydrophones and more particularly to a broad band velocity hydrophone.

United States Patent Leon Apr. 9, 1974 BROAD BAND HYDROPHONE Primary Examiner-Richard A. Farley {75] Inventor: Robert Leon, Glenside, Pa Attorney, Agent, or Firm-G. J. Rubens; Henry Hansen; J. C. Squ1llaro [73] Assignee: The United States of America as represented by the Secretary of the 57 ABSTRACT Navy washmgton A low frequency broad band hydrophone with orthogonal dipole outputs comprising a mass controlled i Filed? Dec- 21, 1966 housing dimensionally much smaller than a wave- 12'} AppL No: 605,128 lengthover the desired frequency band and having four piezoelectric elements cantilever-mounted radially to a center mass suspended within the housing is [52] U.S. Cl. 340/10, 310/84 described, The velocity amplitude of the housi vi. [5] 1 Int. Cl H04! 17/00 bration is both independent of frequency and in phase Field Of Search with the acoustic pressures; the detection of said velocity amplitude by said elements provides orthogonal [56] References Cited 3 fi 'd b d h i b f t d e inven ion escri e erem may e manu ac ure UNITED STATES PATENTS and used by or for the Government of the United f; gf States of America for governmental purposes without lemer 2380.333 M1959 Dranetz 310/8! X the payment of any royalties thereon or therefor. 2,984.111 5/1961 Kritz l 310/84 X Thi invention relates to hydrophones and more 3283590 11/1966 Shang 31018.4 UX particularly to a broad band velocity hydrophone.

' 12 Claims, 6 Drawing Figures 14 BOb 20 Illl r JATENTEDAPR 91m 3.803.546

160 200 FHEOUE/VC Y (a YCL 53) l VOLTAGE (db) m INVENTOR.

ROBERT LEON BY m ATTORNEY BROAD BAND HYDROPHONE BACKGROUND OF THE INVENTION Those concerned with the development of hydrophones have long recognized the need for obtaining usable directional information from a single, small sized hydrophone over a broad, low frequency band. Pressure sensitive hydrophones constructed according to prior art principles would have to be extremely large to exhibit any appreciable directivity index at very low frequencies. For this reason, often several omnidirectional hydrophones are dropped in the water at widely separated precise locations and from phase comparisons between their outputs, the direction of a sound source may be determined. This is both complicated and a time consuming process with many opportunities for error.

While pressure gradient hydrophones yield dipole pattern outputs, their sensitivity is a direct function of their effective dimensions and frequency, thereby providing a 6 db per octave response resulting in low sensitivity for a small size hydrophone, especially at low frequencies. If an amplifier with 6 db per octave is used to compensate for the hydrophone response, a greater amplification of the electrical noise at the low frequencies results, thereby limiting the low frequency operation. In addition, the output of this type hydrophone is 90 degrees out of phase with acoustic pressure below resonance, thereby causing a phase shift network to be required for the formation of cardioid patterns when combined with an omnidirectional hydrophone.

SUMMARY OF THE INVENTION The present invention overcomes the disadvantages of the prior art by using a rigid circular cylinder, suspended vertically in the water such that when acted upon by a plane sound wave, the cylinder vibrates horizontally at the frequency of the sound wave and along the direction of propagation of the wave. By mass controlling the cylinder, the velocity amplitude of this vibration is made independent of frequency and in phase with acoustic pressure. By spacing four stiffness controlled sensing elements radially at 90 degree intervals about an inertial mass suspended within the cylinder and fluid coupling the elements to the housing, the velocity amplitude of the vibration is detected in two mutually perpendicular directions thereby exhibiting cross dipole patterns and providing two outputs, each with a sensitivity that is independent of frequency over a wide frequency band and substantially in phase with the acoustic pressure over this band. One of the outputs is proportional to the cosine of an angle between the direction of propagation of the sound wave and a given horizontal reference line passing through the vertical axis of the hydrophone and the other output is proportional to the sine of this angle. By virtue of this relationship, the direction of the sound wave may be accurately determined.

As a result of this type construction, the cross dipole patterns exhibit vertical signal rejection and accordingly eliminate the majority of surface induced noise. Also, since the outputs from the piezoelectric elements are in electrical phase, a direct comparison may be made over the entire frequency band with a high frequency omnidirectional signal to form cardioid patterns without the need for complex phase shift networks. Additional advantages flowing from this type construction are the frequency independent outputs which make possible broad band frequency operation and eliminate the need for the use of an amplifier with 6 db per octave response which would greatly increase the electrical noise figure at low frequencies. Additionally, the frequency response of the hydrophone falls off at the rate of 12 db per octave below the low frequency resonance of the housing, mass and connecting rods system thereby reducing vibration sensi tivity at the very low frequencies.

An object of the invention is therefore to provide a directional information from a single, small size hydrophone over a wide frequency band, commencing with very low frequencies wherein the outputs are proportional to acoustic pressure and independent of frequency over this band with each output being proportional to the sine and cosine of an angle between the direction of propagation of a sound wave and a given horizontal reference line passing through the vertical axis of the hydrophone.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a side elevational view of an embodiment of the invention;

FIG. 2 shows a section of the device taken on the lines 22 of FIG. 1 looking in the direction of the arrows;

FIG. 3 is an enlarged view of a portion of the invention;

FIG. 4 is an electrical schematic diagram of the embodiment of FIG. 1; and

FIG. 5 is a typical sensitivity vs. frequency response curve for the embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown an embodiment of the invention having a housing assembly 10 comprising a top cover plate 12 of annular configuration and a circular cylindrical shell 14 forming the sides of said assembly. A bottom plate 16 is securely held to the assembly by partially evacuating the gases from the interior of the housing assembly. To insure that the housing assembly is sealed, O-rings 18 and 20 are provided along the periphery of the mounting plate 12 and along the periphery of the bottom plate 16. The top plate is secured to the cylindrical shell 14 by conventional fastening techniques, not shown. The housing assembly is both mechanically and electrically connected to a cable, from which the hydrophone is suspended vertically in the water, as will be described hereinafter, by means of a connector 22 affixed to the cover plate 12.

The interior of the housing comprises an inertial mass assembly 24 with a dense center mass 26 having four composite bilaminate piezoelectric elements 28a-28d located every degrees around the center mass; the elements are cantilever mounted to the center mass by a support 30 having horizontal members 30a connected to the mass and by vertical members 30b which support the bilaminate elements. Each bilaminate element, as

the name implies, is actually two piezoelectric transducing elements physically positioned side by side for providing mechanical to electrical transduction. Such elements are well known to those skilled in the art and are commercially available under the trade name Bl- MORPH by Clevite Corporation. Accordingly, only those parameters which are of interest herein will be described in any detail.

Bilaminate piezoelectric elements 28a-28d are bonded to and electrically insulated from the vertical support members 30b by epoxy cement or other appropriate bonding means and are spaced apart from the inside housing wall by a small amount. This small gap between the piezoelectric element and the inside housing wall is filled with a high viscosity thixotropic paste 40 for transferring the vibration of the housing assembly to the piezoelectric elements as will be described with reference to FIG. 3.

The bilaminate element 28a is electrically connected in series additive relation for bending stresses with the opposite complimentary element 280 thereby producing twice the output voltage. This electrical arrangement is illustrated in FIG. 4a for the elements 28a and 280. FIG. 4b illustrates a similar arrangement for the elements 28b and 28d. This series connected relationship provides for voltage addition for a bending stress in the same direction and for voltage subtraction for opposite bending stresses.

Reference is now made to FIG. 2 which is a sectional view of the hydrophone taken along the line 22 and illustrates how the inertial mass assembly 24 is supported by a mounting plate 36 suspended from the cover plate 12 by four spaced apart vertical rods 32 located every 90 degrees and displaced 45 degrees from the bilaminate piezoelectric elements 28a-28d. Set screws 34 clamp these rods at one end to the cover plate 12 and at the other end to the inertial mass support 36 so that the length of the vertical rods may be adjusted. The rods permit only radial motion of the inertial mass relative to the housing. Therefore, only one mechanical stop 38, machined as part of the cover plate 12, is necessary to keep the bilaminate elements from hitting the inside wall of the housing assembly.

Referring now to FIG. 3, there is shown an enlarged view of the circled portion of the piezoelectric element and mounting structure of FIG. 2. The bilaminate element 28b is made up of two sections, 28b and 28b" separated by a metal shim 42 which electrically connects the two sections in series. The gap between the inner wall of the cylindrical shell 14 and the edge of the bilaminate element 28b is filled with a high viscosity thixotropic paste 40 for transferring the shell vibration to the piezoelectric element. The other end of the element is connected to and electrically insulated from the support 30b by epoxy cement 44.

The transformation of a sound wave to an electrical signal by the apparatus described herein can be best illustrated by the following example. Assume that the direction of propagation of a sound wave is along an axis passing through bilaminate elements 280 and 280, referred to as the X-axis. As a result of this plane sound wave of frequency w and pressure amplitude P, the instantaneous pressure p at any point along the cylindrical body can be defined by the following equation:

p=P Jw(l+.r/c) where x is equal to the distance along the X-axis from the center of the housing and c is equal to the propagation of sound in the liquid medium. On each differential area, the pressure p exerts a differential force component in the X-direction. By using well known mathematical principles, the total force acting on the cylinder in the direction of propagation can be calculated for wavelengths substantially larger than the radius of the cylinder and is found to be:

where H is equal to the height of the cylinder, a is the radius of the cylinder, c is the rate of propagation of sound in the fluid medium. Although the pressure may exert a differential force component in the Y-direction (along a horizontal axis normal to the direction of propagation) on each differential area, integration around the cylinder shows that the net total force acting on the cylinder in the Y-direction is zero. Thus, from equation (2) it can be seen that the plane sound wave exerts a net force on the housing in the direction of propagation proportional to the frequency and degrees ahead of the acoustic pressure measured at the axis.

To a first approximation, the housing and inertial mass may be represented for mathematical purposes, as a mass M suspended from a cable having a restoring force k, which the cable exerts on the housing per unit displacement X Suspended from this mass M via connecting rods having a restoring force K, which the connecting rods exert on the housing per unit differential displacement (X,-X is the inertial mass assembly m. In parallel with the connecting rods is the force C,, which the viscous paste exerts on the inertial mass per unit differential velocity (X -X C,,, is equal to twice the absolute viscosity, u, of the paste times the edge cross sectional area, S, of the bilaminate element divided by the gap, d, between the bilaminate element and the housing wall. The equations of motion of this system are readily obtainable by those skilled in the art and will not be described herein. By solving the equations of motion for the housing assembly and the inertial mass assembly, and assuming that the restoring force of the cable is considerably less than restoring force of the connecting rods, then the motion of the hydrophone as a two degree of freedom system acted on by a force can be defined.

Assuming Newtonian behavior for the viscous paste, the shear force f that the paste exerts on the edge of each bilaminate element is equal to one half C times the component of differential velocity normal to the plane of the element. At low frequencies, the flexural velocity of the bilaminate elements can be neglected in comparison to that of the inertial mass and thus the differential velocity across the viscous paste is simply the difference in velocities of the housing and the inertial mass. if the cantilevered bilaminate element vibrates as a linear, undamped second order system having a resonant frequency w,,, and if 0 is the angle between the direction of propagation and the normal to the plane of one pair of bilaminate elements and if the mass of the housing assembly is equal to the mass of the inertial assembly, then the shear force acting on one of these bilaminate elements in the X-direction can be defined by the following mathematical equation:

77 vraPHP S cos 66 fax Similarly, the shear force acting on an element of the other pair in the Y-direction can likewise be defined by the following equation:

1ra H P S cos Ge fay 21ra g HPuLe cos 0 and 21ra g HP Le sin 0 where each symbol is defined in Table 1.

This analysis has considered only a plane sound wave propagating horizontally. For a plane wave propagating at some angle d) to the horizontal, the effect is to multiply the previously derived force acting on the cylinder by cosine 1). Therefore, it can be easily shown that the above voltage expressions should also be multiplied by cosine 4).

In operation the hydrophone is suspended vertically in the water by a cable attached to a surface station. The housing assembly of the hydrophone, being dimensionally much smaller than a wavelength over the desired frequency band and mass controlled, when subjected to a plane sound wave, vibrates horizontally at a frequency equal to that of the sound wave and along the direction of the wave propagation. As described previously, the velocity amplitude of this induced vibration is independent of frequency and in phase with the acoustic pressure. This vibration is coupled to the bilaminate piezoelectric element by the high viscosity paste 40, which being sheared between the housing wall and the inertial mass assembly, exerts a frequency independent force on both bodies along the direction of propagation. in order to eliminate the possibility of a frequency dependent force from acting on the elements due to standing waves within the housing, it is necessary that only the previously mentioned gap areas contain the viscous fluid.

The force component normal to the plane of the piezo-electric elements bends the elements and thus generates a voltage across them which is proportional to the cosine of the angle between the element normal and the direction of propagation. As described previously, the generated voltage is independent of frequency and in phase with the acoustic pressure by virtue of the stiffness control design of the elements. Accordingly, from the sine and cosine functions relating these two outputs, the direction of the sound source may be computed.

Typical parameters for a hydrophone designed to operate between the 20 and 1,200 cycle per second range are illustrated in Table 1 below.

TABLE 1 a 2.25 inches radius of housing assembly d .040 inches gap between bilaminate element and housing wall 1 .024 inches thickness of bilaminate element h 3.0 inches height of bilaminate element H 4.2 inches height of housing assembly L .595 inches cantilevered length of the bilaminate element p. 5.5 X 10 poises absolute viscosity of the paste M .80 kilograms Mass of housing assembly m .80 kilograms mass of inertial mass assembly K 2,030 Newtons/meter Restoring force connecting rods exert on housing per unit differential displacement g 9.1 X 10 volt meters/Newton piezoelectric constant relating stress in the 1 direction to electric field in the (3) direction E 7 X 10 Newtons/M Modulus of elacticity of bilaminate element p 8.0 grams/CM density of bilaminate element mental mechanical resonance 0 1500 m/sec. velocity of propagation in salt water Output capacitance .02 X 10' farads two serially connected elements S Edge cross sectional area of the bilaminate elements The performance characteristics for ahydrophone having the parameters of Table l are illustrated in FIG. 5 by a plot of sensitivity versus frequency in which the sensitivity is shown by voltage in decibels referred to one volt for a sound field for one microbar. From this curve it can be readily appreciated that the frequency response is relatively flat over the frequency range of interest.

Although a specific embodiment of the invention has been illustrated, numerous modifications and alterations may be made therein without departing from the spirit of the invention. For example, if a higher frequency response characteristic is desired without affecting the low frequency characteristic, by reducing the cantilevered length of the bilaminate element by one half, the resonant frequency will be increased by a factor of 4 while only reducing the sensitivity by fifty percent.

If a smaller radius hydrophone is desired without affecting the sensitivity, this may be accomplished by maintaining the a /M ratio constant in equations (5) and (6).

In addition to the aforementioned improvements in performance characteristics, several improvements in the construction of the hydrophone are possible. For example, the horizontal and vertical support members may be eliminated and the bilaminate elements mounted directly to the center mass, thereby resulting in a reduction in the diameter of the hydrophone. Also, the four vertical rods may be replaced by three shorter ones spaced about the center mass. Further, the bottom plate may include a high frequency omnidirectional hydrophone if desired.

The invention thus described, provides frequency independent outputs making possible broad band operation. The need for compensating amplifiers which greatly increase electrical noise at the low frequencies is also eliminated. Since the invention provides a 12 db per octave response in the region below the natural frequency of the housing, inertial mass and connecting rods system, the vibration sensitivity at very low frequencies is further reduced. The invention also provides two orthogonal dipole outputs which are substan' tially in phase with the acoustic pressure over the frequency band that encompasses frequency independent sensitivity.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

l. A hydrophone for detecting underwater sound sources comprising:

a mass controlled housing assembly;

an inertial mass positioned within said assembly and supported therefrom;

a stiffness controlled transducer operatively connected between a wall of said assembly and said internal mass for detecting the relative motion therebetween; and

a viscous fluid positioned intermediate said wall of said housing assembly and the confronting surface of said transducer for causing said transducer to flex in the presence of relative motion between said housing and said inertial mass.

2. A hydrophone as recited in claim 1 wherein said transducer is one of a plurality of transducers each cantilever mounted radially to said inertial mass and having said viscous fluid interposed between said assembly and the confronting surface of said transducer for detecting the velocity amplitude of the housing vibration in the presence of a sound source, said transducers arranged to provide orthogonal dipole output patterns.

3. A hydrophone as recited in claim 2 wherein said transducers comprise:

bilaminate piezoelectric elements located every 90 degrees around said inertial mass, oppositely located elements connected in electrical series addition for bending motion in the same direction and series subtracting for opposite bending motion.

4. A hydrophone as recited in claim 3 wherein said housing assembly is dimensionally much smaller than a wavelength over a frequency band of said sound sources.

5. A hydrophone for detecting underwater sound sources comprising:

a mass controlled housing assembly dimensionally smaller than a wavelength over a frequency band of the sound sources;

a plurality of rods connected to and extending within said housing assembly;

a mounting plate arranged within said housing assem bly and connected to said rods in spaced relation to said housing assembly;

an inertial mass positioned within said assembly and supported upon said mounting plate;

bilaminate piezoelectric transducer elements cantilever mounted radially every degrees to said inertial mass, oppositely located elements being connected in electrical series addition for bending motion in the same direction and series subtracting for opposite bending motion, said elements further being arranged to provide orthogonal dipole output patterns; and

a viscous fluid interposed between said housing assembly and confronting surfaces of said transducer elements for causing said elements to flex in the presence of relative motion between said housing assembly and said inertial mass for detecting the velocity amplitude of the housing vibration in the presence of a sound source.

6. A hydrophone as recited in claim 3 wherein said housing assembly comprises:

a circular cylindrical shell having a top and bottom plate forming a sealed chamber;

a mounting plate for supporting said inertial mass;

and

a multiplicity of rods connected to said top plate and said mounting plate for suspending said inertial mass in said chamber.

7. A hydrophone as recited in claim 6 further comprising:

a plurality of support members each extending radially from said inertial mass for positioning a respective one of said elements intermediate the wall of said housing and said inertial mass.

8. A hydrophone as recited in claim 7 further comprising:

means electrically insulating said elements from said support members and the wall of said housing.

9. A broadband hydrophone for detecting underwater sound sources comprising:

a mass controlled housing dimensionally smaller than a wavelength over the frequency band of said sound sources whereby the velocity amplitude of the housing vibration caused by said sound source is substantially independent of frequency and in phase with the acoustic pressure of said sound source;

an inertial mass suspended centrally within and from said housing;

a plurality of sensors stiffness controlled over said frequency band, said sensors each being radially cantilever mounted to said inertial mass and having an edge adjacent a wall of said housing;

a thixotropic paste disposed between each of said edges of said sensors and said wall of said housing whereby the relative motion between said housing and said inertial mass shear said fluid and the force created thereby flexes the sensors and produces an output signal proportional thereto.

10. A hydrophone as recited in claim 9 wherein said sensors comprise:

frequency and in phase with the acoustic pressure over said frequency band of said sound sources.

12. A hydrophone as recited in claim 11 further comprising: a mounting plate for supporting said inertial mass; and a plurality of rods connected to said mounting plate and said housing for suspending said inertial mass within said housing. 

1. A hydrophone for detecting underwater sound sources comprising: a mass controlled housing assembly; an inertial mass positioned within said assembly and supported therefrom; a stiffness controlled transducer operatively connected between a wall of said assembly and said inertial mass for detecting the relative motion therebetween; and a viscous fluid positioned intermediate said wall of said housing assembly and the confronting surface of said transducer for causing said transducer to flex in the presence of relative motion between said housing and said inertial mass.
 2. A hydrophone as recited in claim 1 wherein said transducer is one of a plurality of transducers each cantilever mounted radially to said inertial mass and having said viscous fluid interposed between said assembly and the confronting surface of said transducer for detecting the velocity amplitude of the housing vibration in the presence of a sound source, said transducers arranged to provide orthogonal dipole output patterns.
 3. A hydrophone as recited in claim 2 wherein said transducers comprise: bilaminate piezoelectric elements located every 90 degrees around said inertial mass, oppositely located elements connected in electrical series addition for bending motion in the same direction and series subtracting for opposite bending motion.
 4. A hydrophone as recited in claim 3 wherein said housing assembly is dimensionally much smaller than a wavelength over a frequency band of said sound sources.
 5. A hydrophone for detecting underwater sound sources comprising: a mass controlled housing assembly dimensionally smaller than a wavelength over a frequency band of the sound sources; a plurality of rods connected to and extending within said housing assembly; a mounting plate arranged within said housing assembly and connected to said rods in spaced relation to said housing assembly; an inertial mass positioned within said assembly and supported upon said mounting plate; bilaminate piezoelectric transducer elements cantilever mounted radially every 90 degrees to said inertial mass, oppositely located elements being connected in electrical series addition for bending motion in the same direction and series subtracting for opposite bending moTion, said elements further being arranged to provide orthogonal dipole output patterns; and a viscous fluid interposed between said housing assembly and confronting surfaces of said transducer elements for causing said elements to flex in the presence of relative motion between said housing assembly and said inertial mass for detecting the velocity amplitude of the housing vibration in the presence of a sound source.
 6. A hydrophone as recited in claim 3 wherein said housing assembly comprises: a circular cylindrical shell having a top and bottom plate forming a sealed chamber; a mounting plate for supporting said inertial mass; and a multiplicity of rods connected to said top plate and said mounting plate for suspending said inertial mass in said chamber.
 7. A hydrophone as recited in claim 6 further comprising: a plurality of support members each extending radially from said inertial mass for positioning a respective one of said elements intermediate the wall of said housing and said inertial mass.
 8. A hydrophone as recited in claim 7 further comprising: means electrically insulating said elements from said support members and the wall of said housing.
 9. A broadband hydrophone for detecting underwater sound sources comprising: a mass controlled housing dimensionally smaller than a wavelength over the frequency band of said sound sources whereby the velocity amplitude of the housing vibration caused by said sound source is substantially independent of frequency and in phase with the acoustic pressure of said sound source; an inertial mass suspended centrally within and from said housing; a plurality of sensors stiffness controlled over said frequency band, said sensors each being radially cantilever mounted to said inertial mass and having an edge adjacent a wall of said housing; a thixotropic paste disposed between each of said edges of said sensors and said wall of said housing whereby the relative motion between said housing and said inertial mass shear said fluid and the force created thereby flexes the sensors and produces an output signal proportional thereto.
 10. A hydrophone as recited in claim 9 wherein said sensors comprise: bilaminate piezoelectric elements located every 90 degrees around said inertial mass, oppositely located elements connected in electrical series addition for bending motion in the same direction and series subtraction for uniform tension and compression forces.
 11. A hydrophone as recited in claim 10 wherein the flexural displacement amplitude of said elements and the generated voltage therefrom are independent of frequency and in phase with the acoustic pressure over said frequency band of said sound sources.
 12. A hydrophone as recited in claim 11 further comprising: a mounting plate for supporting said inertial mass; and a plurality of rods connected to said mounting plate and said housing for suspending said inertial mass within said housing. 